How to win the rebellion in Poland 1066

On your first question: disinheriting your heir is not possible unless you have elective succession. Then you change for example choose to elect your second or third son. Most of the time your vassals will elect the person you elect. Off course there is a chance they will elect someone else, so it's tricky.

On your second question: yes, it is possible. I have done it before (horrible me) when I was in kinda the same situation as you. My heir died relatively young and so his only son became my heir, but had really bad traits. So I plotted against my grandchild (who was about 4 years old) and managed to successfully kill him. There is a catch however, which happened to me. The plot was successful but the truth came out that I did it, so I got the Dishonorable trait and, even worse, the Kinslayer trait which gave me massive relationship penalty to my family and vassals. So yes, it's definitely possible but there are some serious risks.

You can only get Zealous as an adult via events, which are never guaranteed to fire. You can go on a pilgrimage or hope for an event while educating your ward.

To expand though, you're probably going to have to do it the old-fashioned way: use your Chancellor to forge claims on neighbouring titles, or marry members of your dynasty into titles that their children can inherit later, and then press them in war. Expansion via conquest is really easy but it spoils you like candy: it makes the normal, slow, and strategic expansion that is the mainline gameplay of CKII seem really, really slow. However, that's as it should be — every ruler plays by the same rules, and that slow expansion protects the stability of your borders as much as it prevents you from biting off big chunks of your own neighbours' realms.